In a resist ink to form a protective film or the like on a conventional wiring board, an inorganic filler has been used as a means of improving screen printability (prevention of bleeding during screen printing) in order to meet the tendency of the fining of pitch, as disclosed in Japanese Patent Laid-Open Publication No. 2003-113338 (patent document 1) and Japanese Patent Laid-Open Publication No. 2008-214413 (patent document 2). However, there is a problem that impurities and reaggregation products derived from the inorganic filler become defects to thereby lower electric insulation properties of a resist. By the way, as a method to form a resist on a wiring board, screen printing has been generally adopted.
As a means to achieve a good balance between improvement of screen printability for meeting the fining of pitch and that of electrical insulation reliability, there was a technique to incorporate a special filler into a resist ink, as disclosed in Japanese Patent Laid-Open Publication No. 2007-100038 (patent document 3).
However, it is predicted that with development of a semi-additive process, the line space of wiring of a flexible wiring board is further narrowed (e.g., not more than 20 μm pitch).
With this further fining of pitch, further development of a resist ink (curable composition) excellent in prevention of bleeding during screen printing is desired.
On the other hand, concerning polyurethane having a functional group necessary for curing reaction and a carbonate bond, which is used for a resist, Japanese Patent Laid-Open Publication No. 2003-198105 (patent document 4) discloses a curable composition containing polyurethane having an acid anhydride group and/or an isocyanate group, and a carbonate bond. Moreover, as a curable composition containing polyurethane having a carboxyl group and a carbonate bond, Japanese Patent Laid-Open Publication No. 2006-117922 (patent document 5), Japanese Patent Laid-Open Publication No. 2007-39673 (patent document 6) and Japanese Patent Laid-Open Publication No. 2008-201847 (patent document 7) disclose a curable composition. In any of those documents, however, prevention of bleeding during screen printing is not described at all.